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excerpted from book "Graphics
Programming with GDI+".
GDI+ currently has no support to raster
operations. When we use R2_XOR pen operations, we use the Graphics.GetHdc()
method to get the handle to the device context. During the operation when your
application uses the HDC, the GDI+ should not draw anything on the Graphics
object until the Graphics.ReleaseHdc method is called. Every GetHdc call must be
followed by a call to ReleaseHdc on a Graphics object, as in the following
snippet:
IntPtr hdc1 =
g1.GetHdc();
//Do
something with hdc1
g.ReleaseHdc
(hdc1);
g2 = Graphics.FromImage (curBitmap);
IntPtr hdc1 = g1.GetHdc();
IntPtr hdc2 = g2.GetHdc();
BitBlt (hdc2, 0, 0,
this.ClientRectangle.Width,
this.ClientRectangle.Height,
hdcl, 0, 0, 13369376);
g2.DrawRectangle (Pens.Red, 40, 40, 200, 200);
g1.ReleaseHdc (hdcl);
g2.ReleaseHdc (hdc2);
If we make a GDI+ call after GetHdc, the system
will throw an "object busy" exception. For example, in the preceding code
snippet we make a DrawRectangle call after GetHdc and before ReleaseHdc. As a
result we will get an exception saying, "The object is currently in use
elsewhere."
Using GDI on a GDI+ Graphics Object Backed by a Bitmap
After a call to GetHdc, we can simply call a Graphics object from a bitmap that
returns a new HBITMAP structure. This bitmap does not contain the original
image, but rather a sentinel pattern, which allows GDI+ to tract changes to the
bitmap. When ReleaseHdc is called, changes are copied back to the original
image. This type of device context is not suitable for raster operations because
the handle to device context is considered write-only, and raster operations
require it to be read-only. This approach may also degrade the performance
because creating a new bitmap and saving changes to the original bitmap
operations may tie up all your resources.
